Radiation detector



w. R. BAKER 2,474,773

June 28, 194 9.

RADIATION DETECTOR Filed Nov. 6, 1947 N a I 2-z1 Pulse 23 T 24 002 INVENTOR.

W/LL/AM 1Q BAKER A TTORNEY Patented June 28, 1949 UN?! STATES PATENT F FTCE 2,474,173 itAiJIA'rioN DETECTOR WilliamR,;Baker,-lerkeley, Califi, assignortosthe ,United States of Arnericaas represented by the United States Atomic Energy Commission Application November 6, 1947, Serial No. 784,356 3 Claims. (01. 25043.6)

This invention relates to a circuit network and more particularly to an, amplifier circuit network.

It is r esed by t is nr t'icn t9 Provide a cathode follower type preamplifier tube with a very high effective input impedance and a rela tive 1 t ewnstentthas. b qun t 1 12 .5 type r uit i ext emely. .1 ll in c mb ation with an ionization chamber for the detection q a i tien- Therefore, an object of this invention is to provide a new and improved amplifier network.

Another object of this invention is to provide an amplifier network having a very high effective input impedance and a relatively low time constant.

A further object is to provide an ionization chamber in combination with an amplifier network for radiation detection.

Other objects and advantages of the invention will be apparent in the following description and claims considered together with the accompanying drawing in which:

Figure 1 is a schematic wiring diagram embodying the invention and is the sole figure thereof.

Referring to Fig. 1 in detail, the numeral II indicates a grounded shield generally of cylindrical shape defining a chamber which is substantially free of electrostatic fields and which is provided with an aperture I2 and a cooperating cover plate I3 for the aperture I2. A second chamber I4 is centrally disposed within the chamber I I and is provided with a grid faced aperture I 6 which is in alignment with the aperture I2. The two chambers I I and I4 are electrically connected by means of a high value resistor IT. A collector plate I8 is positioned within the chamber I4 parallel to the grid aperture I6 and in alignment therewith. The collector plate I8 is connected to the control grid of a pentode tube I9 which is disposed within the chamber I4 as is its associated network. The cathode of this tube I9 is electrically connected through a cathode bias resistor 2I which is shunted by a condenser 22 and a resistor 23 which is shunted by a condenser 24 to one wall of the chamber I4. The junction point of the resistors 2| and 23 is connected through a high value grid resistor 26 to the control grid of the tube I9. An internal connection is made between the suppressor grid and the cathode of the tube l9, whereas the screen grid is connected through a condenser 21 having a large capacitance to the junction point between the resistors 2I and 23. A further connection is made from the screen grid to the plate of the By the arrangement of thecircuit elements, the

effective input impedance oflthe amplifier .may be expressed as being equal .to R'gil-rRkGm), where Rg is thevalue of the resistor 26, -Ri; is the value of the resistor I1, and Gmisthe ilalueo'f the transconductance of the tube l9. Thus, if a tube is chosen which has a value of transconductance of about 5000 micromhos, if the resistor 26 has a value of about megohms, and if the resistor I! has a value of about 10 megohms, it is seen that an effective input impedance of about 5 million megohms is attained. Also, the input time constant of the amplifier circuit may be expressed in terms of the circuit elements; that is, the input time constant may be expressed as the product of the value of the resistance 26 and the input capacitance of the tube I9. The input capacitance of the tube I9 is reduced by the large capacitive coupling condenser 21 between the screen grid and the cathode which substantially follows the control grid. Likewise, the input capacitance of the tube I9 is reduced by connecting the aperture grid I 6 to the cathode of the tube through the condensers 22 and 24.

In operation, a radioactive sample is placed on the cover plate I3 and exposed to the interior of the chamber II. The charged particles accelerated through the grid aperture I 6 to the collector plate I8, produce a voltage drop across the resistor 26 which results in an output voltage substantially equal to the product of the input cur rent and the eifective input impedance. The current as well as the duration of the pulse may be determined by the field between collector plate I8, the grid aperture I6 and the walls of the chamber I I, by the energy and the type of particle emitted by the radioactive sample, and by the type of gas filling the ionization chamber. The duration of the pulse may also be determined by the input time constant of the amplifier circuit.

While the salient features of this invention have been described in detail with respect to one embodiment, it will, of course, be apparent that numerous modifications may be made within the spirit and scope of this invention and therefore it is not desired to limit the invention to the exact details shown except in so far as they may be defined in the following claims.

What is claimed is:

1. In a radiation detector, the combination comprising a first chamber having a port formed therein, a cover for said port adapted to support radioactive material and to be sealed over said port, a second chamber disposed Within said first chamber having a grid portion aligned with said port, a collector plate disposed within said second chamber and aligned with said grid and port, electrical means connected to said first and second chambers and said collector plate for maintaining said second chamber at a positive potential with respect to said first chamber and said collector plate at a positive potential with respect to said second chamber, and an amplifier disposed Within said second chamber and connected between said second chamber and said collector plate for amplifying the charge collected by said collector plate.

2. In a radiation detector, the combination comprising a substantially cylindrical chamber having a port formed therein, a cover for said port adapted to support radioactive material on the inner surface thereof and to be sealed over said port, a second chamber centrally disposed Within said cylindrical chamber having a grid portion aligned with said port, a collector plate disposed Within said second chamber and aligned with said grid and port, an amplifier tube having at least anode, control grid, and cathode circuits disposed within said second chamber, said cathode circuit comprising a tapped first resistor and 4 a second resistor, said first resistor being connected between said cathode and said second chamber and the tapped point connected to said control grid and said collector plate through a grid resistor, said second resistor connected between said cylindrical chamber and said second chamber, condenser means connected in parallel with said first resistor for by-passing varying voltages developed thereacross, and potential means connected between said anode circuit and said cylindrical chamber.

3. In a radiation detector of claim 1, said amplifier further characterized as a cathode follower having an input and an output circuit, said input circuit being connected to said collector plate, and said output circuit including the potential between said chambers.

WILLIAM R. BAKER.

REFERENCES CITED The following referenlces are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,200,055 Burnett May 7, 1940 2,241,534 Blllmlein et a1 May 13, 1941 2,275,747 Fearon Mar. 10, 1942 2,288,718 Kallmann et al. July 7, 1942 

